Learn Turkish: Çocuklar aç mı? - Are the children hungry?

The yes/no question particle changes based on 4-way vowel harmony, matching the last vowel of the word before it. Because the word aç contains the vowel a, the particle must be mı.

Where is the word for "are" in this sentence?

Turkish does not have a separate word for "am", "is", or "are". Normally, you add personal suffixes to the end of the sentence to show who you are talking about. But for the third person ("they"), the suffix is usually left completely blank. The particle mı alone is enough to ask the question.

Since "children" is plural, shouldn't there be a plural suffix at the end, like açlar mı?

If the subject is a human and already has a plural suffix (like -lar in çocuklar), you do not need to repeat the plural suffix at the end of the sentence. Saying Çocuklar aç mı? is perfectly natural and correct.

Does the position of mı matter? Could I say Çocuklar mı aç?

Yes, the position matters! The question particle asks about the word directly in front of it. Çocuklar aç mı? asks "Are the children hungry?". If you say Çocuklar mı aç?, it means "Is it the children who are hungry?" (as opposed to someone else).